Contact Information

Office of Biological and Environmental Research
U.S. Department of Energy Office of Science

(SC GG 5.21.1)
Develop predictive model for contaminant transport that incorporates complex
biology, hydrology, and chemistry of the subsurface. Validate model through field

Second Quarter Results

New information on biogeochemistry, groundwater and
subsurface media from the Oak Ridge Field Research Center (FRC) was utilized
to update and run large-scale 3-D flow and chemical transport models. The
FRC is a focal point for ERSD (now CESD) program field research on natural and stimulated
biologically-mediated attenuation of metals and radionuclides. Information
from these studies on the interactions among biological, chemical and physical
processes in a real-world setting with complex hydrogeology and contaminant
characteristics promises to significantly improve DOE’s ability to
effectively manage legacy waste sites. Computer models are important tools
for interpreting field and laboratory data from this complex system to understand
the nature of process interactions, to help guide ongoing and future research
efforts, and to enable predictions of plume-scale behavior in response to
various remediation/management strategies.

Field-scale modeling efforts are being conducted by
ORNL, PNNL, Oregon State University and Stanford University researchers on
experimental plots within the FRC and at a larger scale that encompasses
all of the field plots to better understand their interactions and large-scale
behavior. These efforts have utilized the code HYDROGEOCHEM Version 5, which
simulates three-dimensional transient density-dependent, fully-anisotropic
saturated and unsaturated water flow, dissolved transport, and complex equilibrium-
kinetically-limited biogeochemical reactions. Work was undertaken to develop a site-wide 3D flow
and transport model encompassing an area of approximately 280 acres that
includes FRC Areas 1, 2 and 3, the former S3 Ponds and the Bear Creek watershed
from its head waters to the NT2 tributary. Refinements in the model have
been completed to incorporate new data and improvements in the conceptual
model of the site. The refinements include:

Modify the gravel fill zone that extends from slightly
west of Area 3 to Bear Creek to incorporate information from new soil borings
that indicate the fill directly overlays saprolite locally and is more
extensive than previously understood. This may significantly affect shallow
groundwater flow and contaminant transport towards Bear Creek. Modify the areal extent of the rock-saprolite transition
zone based on new drilling results and geophysical testing, which indicate
the zone is not as extensive as previously assumed. Incorporate the permeable barrier trench in FRC
Area 2 in the model.

Add a “potential high permeability zone” in
the bedrock inferred from recent geophysical testing.

The refined site-wide FRC model is discretized into
8 layers of 23,967 elements and 10 layers of 13,680 nodes. Four types of
bedrock, including a “potential high permeability zone” identified
by geophysical testing, are overlain by saprolite, gravel fill, a permeable
trench and a rock saprolite “transition” zone (Figure 1). Nonlinear
optimization using the PEST code was performed to recalibrate a steady-state
groundwater flow model to stream gauging data and water levels from 74 wells.
Rock and saprolite were modeled as anisotropic media with a maximum permeability
along strike, minimum permeability in the cross-bed direction and intermediate
permeability along the dip direction. Field-scale dispersivities and effective
porosities were manually calibrated to measured nitrate concentrations from
19 wells within and near the dissolved plume by simulating non-reactive nitrate
plume evolution from the S3 Ponds considering density-dependent flow effects.
Dissolved nitrate plume predictions are shown in Figure 2. Sensitivity analyses
were performed.A higher resolution model was developed for Area 3
to facilitate interpretation of in situ Area 3 flow and tracer studies in
the vicinity and to design subsequent experiments (Figure 3). A sub-model
domain was delineated and a numerical mesh developed to accommodate the finer
resolution needed to simulate flow and transport within Area 3. Boundary
conditions on the sub-model domain were obtained from the site-wide model.
Material properties were initially mapped from the site-wide model, and then
refined to account for finer level details relevant to the experimental data
interpretation. The fine-scale model will be utilized to assess potential
interactions between current experiments in Area 3 and proposed new studies
and to adjust experimental plans if necessary to avoid adverse effects on
neighboring field plots. Simulated breakthrough curves for a tracer test
in FW106 are shown in Figure 4.A high resolution model was also developed for Area
2 to help design and interpret field-scale experiments in this area (Figure
5). The 800 m3 model domain encompasses the disturbed fill, coarse gravel,
and intact saprolite zones. The model considers pulsed injection of tracers
and ethanol in three wells (FW213, FW212, FW214) and simulates 94 chemical
species, 8 biomass populations, 58 equilibrium reactions, 77 kinetic reactions,
and 37 terminal electron accepting reactions. Reactions and reaction coefficients
and aquifer properties were based on data from field and laboratory studies.
Figure 6 shows comparisons of model predictions and observations at 2.5 m
(MLS-A, MLS-B, FW216) and 7.5 m (FW202) downstream of the injection wells
during the first week. Generally, the model predictions and observations
match reasonably well given that no reaction parameter fitting was performed
for the field observations. The timing and rates of utilization of the various
terminal electron acceptors is approximately correct in the simulation results.
The primary mismatches occur at MLSA-4 and MLSA-5, ports at which the bromide
simulation very poorly matches the observed concentrations. This suggests
that incorrect representation of the flow pathway, probably associated with
uncharacterized physical heterogeneity, is the cause of the mismatch rather
than improper modeling of the reaction system and rates.

Highlights

Please wait

Citation and Credit

Unless otherwise noted, publications and webpages on this site were created for the U.S. Department of Energy Subsurface Biogeochemical Research program and are in the public domain. Permission to use these documents is not needed, but please credit the U.S. Department of Energy Subsurface Biogeochemical Research program and provide the URL http://doesbr.org when using them. Materials provided by third parties are identified as such and are not available for free use.